I have driven X3 2.8ix several times as loaners, just one brief drive in X3 2.8d. The gasser has more than adequate power with just driver aboard, but fuel economy takes a hit compared to 328i, and with a full load I'm not sure it would feel so sprightly. The X3 2.8d was noticeably sluggish compared to my 328xd, could really feel the weight and aero drag, but it wasn't terrible with just two of us aboard and no cargo.
Like yatzee, I'm not impressed with the 328xd seats and seating position. By comparison the X3s had a better seat/pedals/wheel relationship, though the base seats were still rather hard and flat, the seats themselves not superior to the sedan.
My X3 objections are of course the penalty of run-flats, and the value given the high costs to buy and maintain out of warranty. I think the Audi Q5 is a superior vehicle and a better value, which is strange, because I did not not particularly like Q5 when it first came out. Audi has done well refining it and building it, people we know who own them love them and have had no problems.
The M-B GLK diesel is alright, but cramped inside, a little jiggly in the ride motions, and not at all cheap to run. Front seats are okay, but the rear seats are terrible and not much cargo room either.